1. Field of the Invention
This invention relates to an apparatus for examining surfaces of a transparent flat object such as a glass member or a pericle and, more particularly, to an apparatus for detecting foreign particles attached to surfaces of such a flat object. More specifically, the present invention relates to an inspection apparatus capable of discriminating the positions of foreign particle detects with respect to obverse and reverse surfaces of the flat object.
2. Related Background Art
A technique for protecting a reticle (or a mask) used for projection exposure apparatus for manufacturing semiconductor devices is known which technique is based on covering the reticle with a pericle formed of a thin film of a high polymer such as nitrocellulose. The pericle is supported over the reticle by a support frame to cover the reticle while being spaced apart from the surface of the reticle by a certain distance, thereby preventing any foreign particles from attaching the reticle.
The pericle has a thickness of about 1 .mu.m and a transmissivity of 90% or higher with respect to exposure illumination light. The thickness of the support frame is determined so that the stand-off of the pericle from the reticle surface is several millimeters, which is determined in consideration of the depth of focus of the projection optical system of the exposure apparatus on the reticle side.
If IC patterns are projected for exposure on a wafer by using a reticle and a pericle combined in this way, the image of a foreign particle on the reticle surface is transferred onto the wafer while being reduced according to the reduction ratio of the projection optical system, but the image of a foreign particle attached to the pericle is not resolved since it is defocused on the wafer surface. However, if the size of a particle on the pericle is greater than several ten microns, the image of this particle appears on the wafer as a defocused shadow. Accordingly, it is also necessary to check foreign particles attached to the pericle.
A type of inspection apparatus used for this purpose including the ones disclosed in Japanese Patent Publication No. 63-52696 and the one U.S. Pat. No. 4,716,299 is known in which a surface to be inspected is irradiated with a laser beam led at a small angle to this surface to form a stripe-like irradiation region on the inspected surface, and in which a one-dimensional array sensor is disposed in a position at which it can receive side scattered light among the scattered light from the irradiation region. However, according to this conventional technique, it is not possible to discriminate whether a foreign particle is attached to the obverse surface of the pericle (on the light source side) or to the reverse surface of the pericle (on the reticle side).
It is very important in terms of photolithography process to know whether a foreign particle attached to the pericle exists on the obverse side or on the reverse side. If the foreign particle is attached to the reverse surface, there is a risk of the foreign particle moving apart from the pericle and attaching to the reticle and, hence, a risk of occurrence of a defect in a shot on the exposure wafer effected by using this reticle. In such a situation, it is necessary to remove the pericle from the reticle together with the frame, to perform foreign particle removing operation for the reticle alone, and to replace the pericle with a new one.
It is therefore important to correctly know whether or not replacement with a new pericle is necessary before the exposure operation on the actual device.
Japanese Patent Laid-Open No. 63-186132 (corresponding to U.S. Pat. No. 4,889,998) discloses an apparatus for discriminating whether a foreign particle is attached to the obverse surface or the reverse surface of a pericle. This apparatus, however, requires a pair of light sources for drop shot illumination and oblique illumination.
U.S. Pat. No. 4,468,120 discloses an apparatus for discriminating whether a foreign particle is attached to the obverse surface or the reverse surface of a glass substrate. This apparatus requires a pair of photoelectric elements, i.e., a photoelectric element for receiving scattered light produced in a space on the obverse side of the substrate and another photoelectric element for receiving scattered light produced in a space on the reverse side of the substrate. Although the discrimination of the obverse or reverse surface to which a foreign particle is attached may be possible based on the same principle with respect to the pericle alone, it is not possible to inspect the pericle when the pericle is attached to the reticle, because the scattered light is shaded by patterns (chrome layer) on the reticle.